The southern end of the beach near the Gawler river might be suitable for your needs, this way it will also keep away from the ALYC races.
Most days at low tides would give a sailable areas, the warmer the weather the bigger the areas and keep an eye out for a "dodge tide" its where the tide is low for several days (only happens on a couple of spots in the world... and not that often
http://www.bom.gov.au/oceanography/tides/dodge/dodge.shtml )
www.seabreeze.com.au/graphs/sa.aspTake lots of aeroguard.... I will say it again....
Take lots of aeroguardAnother couple of spots that might be worth a try is Largs Bay, north of the sailing club (the kite buggy guys use this spot) or even Goolwa ocean beach head east (toward murray mouth) from the car park.